We have studied partial hydrogenation of 1, 3-butadiene to butene on a hydrogen-precovered Pd(110) surface, focusing on how reactants were catalytically activated on a metal surface. Techniques of surface analysis, including temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM), were utilized to identify chemisorbed states of the active species involved in hydrogenation. For 1, 3-butadiene coverage of lower than 0.25 ML, 1, 3-butadiene was in a strongly π-bonded state on H/Pd(110) and caused some of the precovered hydrogen atoms to move into the subsurface site. Under this condition, a hydrogen exchange reaction between the adsorbed 1, 3-butadiene molecules and precovered hydrogen atoms occurred. At 1, 3-butadiene coverage of 0.25 ML, the 1, 3-butadiene molecules, which formed a c(4 × 2) layer accompanied with subsurface hydrogen atoms, were less active in hydrogen exchange and hydrogenation. In contrast, when 1, 3-butadiene coverage exceeded 0.25 ML, the Pd surface was partially covered with weakly π-bonded 1, 3-butadiene and became active in hydrogenation. The obtained results indicate that hydrogenation of 1, 3-butadiene on Pd(110) requires the presence of both weakly π-bonded 1, 3-butadiene and subsurface hydrogen atoms, and that it is important that the catalytic active species are produced by the reactants themselves. Here, we introduce a new concept of a self-activated catalyst layer for hydrogenation on Pd(110).
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